This invention relates to a multi-function multi-electrode device. In particular, it relates to a multi-function multi-electrode device for medical use.
Defibrillation is used to depolarize cardiac muscle fibres by means of a large electric shock in an attempt to re-establish normal cardiac rhythm. A large current of up to 50 A and several thousand volts is typically applied to a patient, using a damped sinusoidal waveform with a duration of approximately 5-10 ms, or a truncated exponential waveform of 5-30 ms duration.
External cardiac pacing stimulates the heart at regular intervals to enable control of the heart rate when the intrinsic cardiac rate is not sufficient. External cardiac pacemakers apply modest levels of current (50-200 mA) and voltage to a patient, using rectangular or truncated exponential current pulses having durations of 10-40 ms.
Due to their differing operating conditions, electrodes for cardiac defibrillation and external pacing have differed in design. In external pacing, for example, a major problem is that a substantial portion of the applied current flows into the patient via the peripheral areas of the electrode rather than there being a uniform current density over the electrode surface. This is referred to as the perimeter effect and the large local current densities at the periphery can result in serious skin burns and cutaneous pain at the electrode edges. In order to ensure a more uniform current density distribution and thus avoid or minimise the above problems, relatively thick, wide, high-resistance gel pads have been used.
The use of high impedance gel pads, however, results in significantly lower delivered current, and thus may limit the effectiveness of cardiac defibrillation attempts. The passage of large currents through such high impedances gives rise to a significant temperature rise and can be accompanied by spark generation which, in the presence of oxygen, is extremely dangerous.
Fires during resuscitation attempts have been reported in the literature. Low resistance gel pads are therefore commonly used in external defibrillation electrodes.
Two different electrode systems are therefore generally used for cardiac defibrillation and external cardiac pacing. This is less than optimal as two sets of electrodes may be required for a patient. Such a cumbersome arrangement would also tend to be relatively expensive.
In U.S. Pat. No. 4,776,350 (Grossman) there is disclosed an electrode comprising two parallel electrically conductive in-contact members. In order to minimise the current density concentrations around the electrode edge, the first conductive member has an area resistivity much less than that of the second conductive member. The conductive members are composed of a conductive polymer such as a carbon filled vinyl or carbon filled rubber material. The resistivities of the two conductive members depend upon the application to which the electrode is to be put. Hence the electrode may be used as a cardiac pacing electrode or as a cardiac defibrillation electrode.
In a preferred arrangement of the invention described in U.S. Pat. No. 4,776,350, the second conductor member comprises a laminate composed of an electrically conductive rubber sheet, sandwiched between two layers of conductive gum adhesive. The area resistivity of the rubber layer is greater than the area resistivity of each gum adhesive layer. It is believed that current tends to flow preferentially through areas of the poorly mixed gum adhesive which have higher concentrations of conductive substances. It is thought that the incorporation of a layer of a high resistance rubber between layers of conductive gum adhesive will reduce the tendency of this form of current "hot spot" production. The current still has to flow through the bottom layer of gum adhesive with all of its "hot spots". The gum adhesive layers and the conductive layers appear to have the same areas.
The above arrangement gives rise to a very resistive electrode device, potentially suitable for cardiac pacing but less than optimal as a defibrillation electrode.
In "Electrodes and the measurement of bioelectric events", Geores, L. A. Wiley--Interscience, New York, 1972, it is known to provide a "guard-ring" electrode which comprises a pair of coplanar electrodes which are electrically isolated from each other. Essentially, one electrode is located concentrically relative to the other electrode. The outer electrode is used to modify the current density distribution of the inner electrode. Both electrodes are operated at the same potential to enable the inner or smaller electrode to be used for current measurement. The outer electrode, known as the guard electrode aids in providing a uniform current-density distribution around the main current path in the specimen. The current flowing in the guard electrode is not measured. However, a disadvantage of such an arrangement is that the current density of the outer electrode is not uniform and, accordingly, this electrode arrangement is only of benefit if the inner electrode is used for measurement purposes.
It is an object of the present invention to overcome these problems.